1. Field of the Invention
The present invention relates to a radar device for applying a transmission signal (electronic waves) to a target object (hereinafter referred to as “target”), receiving a reflection signal based on the transmission signal from the target by plural receiving units and calculating target information on the basis of the reflection signal.
2. Description of the Related Art
As a radar device for calculating the distance to a target and the relative speed has been hitherto known a frequency-modulation radar device in which a transmission signal having continuously-modulated frequencies is transmitted to a target, a reflection signal from the target is received and the distance to the target and the relative speed are calculated.
A method of mechanically turning a transmission unit to sweep a transmission signal and calculating the direction to a target is known as a method of calculating the target direction in this type radar device. Furthermore, there is also known a super-resolution incoming direction (arrival direction) estimation processing such as Music (Multiple Signal Classification) method of outputting a transmission signal without mechanically turning the transmission unit and executing digital signal processing on a reception signal received by an array antenna comprising plural channels to calculate the direction to a target (for example, R. O. Schmidt “Multiple Emitter Location and Signal Parameter Estimation” IEEE Trans. Ap-34, No. 3, pp. 276-280 (1986); hereinafter referred to as “non-patent document”).
According to the MUSIC method disclosed in the non-patent document, a correlation matrix of a peak frequency spectrum is calculated, the correlation matrix is subjected to an eigendecomposition, an angle spectrum is calculated from an eigenvector and the direction to a target is calculated from the angle spectrum. Furthermore, for example, according to a method disclosed in JP-A-2006-145251 (hereinafter referred to as “patent document”), a reception vector is not generated by using only the peak frequency of a peak waveform, but reception vectors are also generated by using other frequencies belonging to the same peak waveform. In this case, the correlation matrixes thereof are calculated and added, whereby the number of snapshots used to calculate the correlation matrixes is secured.
The super-resolution incoming direction estimation processing which is represented by the technique disclosed in the non-patent document is based on the assumption that the respective incoming waves are irrelevant to one another. Therefore, it cannot be directly applied to land mobile communication or the like in which the correlation among the respective incoming waves is very high. In order to suppress the correlation among the incoming waves, it is generally desired to increase the number of reception signals used to generate the correlation matrix (the number of snapshots).
Accordingly, a correlation matrix is obtained every measurement in the radar device, and thus there is known a so-called time averaging method in which the number of snapshots is secured by utilizing correlation matrixes obtained through past measurements executed over plural periods. With respect to the time averaging method in which the number of snapshots is secured by utilizing the correlation matrixes obtained through the past measurements executed at plural periods, when it is applied to a mobile object such as an in-vehicle mount radar device or the like, the positional relationship with a target is varied in accordance with the measurement timing, and thus the incoming direction of electronic wave is varied. Therefore, when it takes longtime to secure the number of snapshots, the precision of the correlation matrix (and thus the estimation precision of the incoming direction of the electronic wave) is lowered.
On the other hand, according of the patent document described above, in order to secure the number of snapshots, a reception vector is not generated from only the peak frequency of a peak waveform, but reception vectors are also generated from other frequencies belonging to the same peak waveform, and the correlation matrixes thereof are calculated. Therefore, the calculation is complicated, and also the direction to a target cannot be calculated with high precision because the frequencies other than the peak frequency are used.